1. Field of the Invention
This invention relates to an analytical method to provide an optimum design for a monolith washcoat and to the resulting optimum pore structure of a washcoat for a monolith catalyst.
2. Description of the Previously Published Art
Oxidation and three-way emission control catalysts are well known in the prior art and among these are catalysts with specified physical characteristics. Ziezenlair et al in U.S. Pat. No. 3,907,512 disclose a pellet type oxidation catalyst having a pore volume greater than about 1.0 cm.sup.3 /g; a surface area greater than about 275 m.sup.2 /g and a macropore diameter of from 0 to 10,000 Angstrom units. Hegedus et al in U.S. Pat. No. 4,119,571 also disclose a pellet type oxidation catalyst which has a surface area of from 100 to 250 m.sup.2 /g; a pore structure characterized by a macropore volume of at least 0.020 cm.sup.3 /g and a macropore radius of greater than about 10,000 Angstrom units. Neither of these two pellet type catalyst inventions extend such pore structure characterizations to washcoats for monolith type emission control catalysts.
Dupin in U.S. Pat. No. 4,529,718 discloses a washcoat with both macroporosity and microporosity. The washcoat surface area is from 20 to 350 m.sup.2 /g with a macropore volume of at least 0.050 cm.sup.3 /g and a macropore radius greater than about 10,000 Angstrom units. Turner et al in EP No., 119,715 disclose a method for increasing the macroporosity of a washcoat by blending two refractory oxide supports with differing surface areas and pore size distributions. Oh et al in Ind. Eng. Chem. Prod. Res. Dev., 22 p. 509-518 (1983) provide a model for design aspects of poison-resistant automobile monolithic catalysts. Neither the Dupin or Turner inventions nor the Oh et al article, however, provide any algorithm for optimizing the washcoat's pore structure to maximize poison resistance of the catalyst or specify a required washcoat thickness to ensure sufficient catalytic performance over the required lifetime of the catalyst.